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Optical scanning system for surface inspectionOptical scanning system for surface inspection description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060203235, Optical scanning system for surface inspection. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This is a continuation of application Ser. No. 10/412,458, filed Apr. 10, 2003; which application is a continuation of application Ser. No. 09/571,303, filed May 8, 2000, now abandoned; which application is a continuation of application Ser. No. 08/868,292, filed Jun. 3, 1997, now U.S. Pat. No. 6,081,325; which application claims the benefit of the filing date of Application No. 60/018,973, filed Jun. 4, 1996. These applications are hereby incorporated by reference as if fully set forth herein. BACKGROUND OF THE INVENTION [0002] The present invention pertains to the field of optical surface inspection. Specifically, the present invention pertains to illumination and light collection optics for inspecting semiconductor wafers and the like. [0003] Monitoring anomalies, such as pattern defects and particulate contamination, during the manufacture of semiconductor wafers is an important factor in increasing production yields. Numerous types of defects and contamination, especially particles, can occur on a wafer's surface. Determining the presence, location and type of an anomaly on the wafer surface can aid in both locating process steps at which the anomaly occurred and determining whether a wafer should be discarded. [0004] Originally, anomalies were monitored manually by visual inspection of wafer surfaces for the presence of particulate matter. These anomalies, usually dust or microscopic silicon particles, caused many of the wafer pattern defects. However, manual inspection proved time-consuming and unreliable due to operator errors or an operator's inability to observe certain defects. The ever increasing size of the wafer surface, along with the decreasing dimensions of the components thereon, resulted in a sharp increase in the number of components on the wafer's surface. The need for automation became manifest. [0005] To decrease the time required to inspect wafer surfaces, many automatic inspection systems were introduced. A substantial majority of these automatic inspection systems detect anomalies based on the scattering of light. For example, see U.S. Pat. No. 4,601,576 to L. Galbraith, assigned to the assignee of the present invention. These systems include two major components: illumination optics and collection-detection optics. Illumination optics generally consists of scanning a wafer surface with a coherent source of light, e.g., a laser. Anomalies present on the wafer's surface scatter incident light. The collection optics detect the scattered light with reference to the known beam position. The scattered light is then converted to electrical signals which can be measured, counted and displayed as bright spots on an oscilloscope or other monitor. [0006] The illumination optics plays a major role in establishing the detection sensitivity of the inspection system. The sensitivity is dependent upon the size of the spot scanned on the wafer and the illumination angle. The smaller the spot size, the more sensitive the system is to detecting anomalies. However, decreasing the spot size increases the time required to scan the wafer surface and therefore reduces throughput. [0007] The sensitivity of both the illumination and collection-detection optics is dependent upon the texture of the surface of the wafer illuminated. If the surface illuminated is patterned, this reduces the sensitivity of the system because such areas produce scatter which makes it difficult to determine the presence of an anomaly. To abrogate scatter due to patterned features, the angle of incidence of the spot on the surface is increased, with respect to the normal to the surface. However, too great of an angle, i.e., a grazing angle with respect to the surface, will also reduce the sensitivity of the system. Moreover, increasing the angle of incidence, increases the effective size of the spot, thereby reducing the sensitivity of the system. Thus, a trade-off exists between sensitivity and inspection rate of the system. The sensitivity of the collection-detection optics is generally a factor of the detector's azimuthal position with respect to the scanning beam and elevation. [0008] Accordingly, many illumination and collection-detection techniques have been proposed that take advantage of the aforementioned concepts. In addition, efforts have been made to provide for constant scanning of the wafer's surface to further increase the speed of the inspection. In U.S. Pat. No. 5,317,380, Allemand discloses a beam of laser light brought to focus as an arcuate scan line on a surface, at a grazing angle of incidence. A pair of light detectors are provided to collect light which is scattered away from the beam in a forward direction so that the angle of collection is constant over the entire scan line. [0009] U.S. Pat. No. 4,912,487 to Porter et al. discloses a laser pattern writing and inspection system that illuminates a target surface with an argon ion laser beam. An acousto-optical deflector is driven with a chirp signal and placed in the path of the beam to cause it to sweep out raster scan lines. The target is placed on a stage capable of bi-directional movement. The beam has an angle of incidence normal to the target and the stage moves so that it is scanned along adjacent contiguous strips of equal width. [0010] U.S. Pat. No. 4,889,998 to Hayano et al., discloses an apparatus and method for detecting foreign particles on a pellicle using a beam of light that is scanned across the pellicle with light detected by a plurality of detectors grouped in pairs. Two pairs of detectors are positioned to collect rearwardly scattered light. The difference in intensity of scattered light detected by each detector is monitored, whereby the position of the particle on the pellicle is determined by analyzing the intensity variations. [0011] In U.S. Pat. No. 4,898,471 to Stonestrom et al., an apparatus and method for detecting particles on a patterned surface are disclosed wherein a single light beam is scanned, at a grazing angle of incidence, across the surface. The surface contains a plurality of identical dies with streets between them. With the beam scanning parallel to the streets, a single channel collection system detects scattered light from an azimuthal angle that maximizes particle signals while reducing pattern signals. A processor constructs templates from the detected light which corresponds to individual dies and then compares the templates to identify particles on the dies. [0012] In U.S. Pat. No. 4,617,427, Koizumi et al., a wafer is mounted on a feed stage connected to a rotary drive which provides a constant speed helical scan of a wafer surface. An S-polarized laser beam is scanned thereon at varying angles of incidence. The angle of incidence is dependent upon whether the wafer is smooth or patterned. A single detector is positioned perpendicular to the wafer's surface for collecting scattered light and includes a variable polarization filter that attenuates scattered light in an S polarization state, when the surface is patterned, and does not attenuate light if the wafer is smooth. [0013] In U.S. Pat. No. 4,441,124 to Heebner, a laser is scanned over the surface of a wafer at an angle normal thereto. The laser beam is scanned by deflecting it with a galvanometer and an acousto-optic deflector in synchronization with the scanning beam rate of a video monitor. A photodetector employing a ring-type collection lens monitors the intensity of light scattered substantially along the wafer surface. This arrangement was employed to take advantage of the finding that a patterned wafer having no particulate matter thereon will scatter substantially no light along the wafer surface, while a wafer having particulate matter on it will scatter a portion of the light impinging thereon along the surface. [0014] Another particle detection apparatus and method is disclosed in U.S. Pat. No. 4,391,524, to Steigmeier et al., wherein a laser beam is scanned at an angle normal to the wafer's surface. In addition to rotating, the wafer stage is provided with movement along one axis that results in the wafer being scanned in a spiral fashion. A single detector is positioned perpendicular to the surface to collect scattered light. Threshold circuitry is employed to discriminate between the defects monitored. [0015] It is an object of the present invention to provide a high-speed apparatus which is capable of scanning a laser beam across the surface of either a patterned or unpatterned wafer to detect anomalies thereon with sizes on the order of a fraction of a micron. [0016] It is a further object of the present invention to classify detected anomalies and determine their size while increasing the confidence and accuracy of the detection system by reducing false counts. SUMMARY OF INVENTION [0017] These objects have been achieved with an apparatus and method for detecting anomalies of sub-micron size, including pattern defects and particulate contaminants, on both patterned and unpatterned wafer surfaces. For the purposes of this application, a particulate contaminant is defined as foreign material resting on a surface, generally protruding out of the plane of the surface. A pattern defect may be in or below the plane of the surface and is usually induced by contaminants during a photolithographic processing step or caused by crystal defects in the surface. [0018] One aspect of the invention is directed towards an optical scanning system for detection of anomalies, such as particles and pattern defects on a surface, comprising means for directing a focused beam of light onto a sample surface to produce an illuminated spot thereon and means for scanning the spot across the surface along a first scan line. The system further comprises a first detector positioned adjacent to said surface to collect scattered light from the spot wherein the detector includes a one-dimensional or two-dimensional array of sensors and means for focusing scattered light from the illuminated spot at each of a plurality of positions along the scan line onto a corresponding sensor in the array. [0019] Another aspect of the invention is directed towards an optical scanning method for detection of anomalies, such as particles and patterns on a surface, comprising the steps of directing a focused beam of light onto a sample surface to produce an illuminated spot thereon; scanning a spot across the surface along a first scan line; positioning a first detector adjacent to said surface to collect scattered light from the spot, wherein the detector includes a one-dimensional or two-dimensional array of sensors; and focusing scattered light from the illuminated spot at each position along the scan line onto a corresponding sensor in the array. BRIEF DESCRIPTION OF THE DRAWINGS [0020] FIG. 1 is a simplified perspective plan view of the illumination and collection optics of the present invention. Continue reading about Optical scanning system for surface inspection... 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